The present invention relates to ink jet printing and, more particularly, to an improved scheme for mechanically stimulating ink fluid filaments to cause the filaments to break up into drop streams of substantially uniform drop size and interdrop spacing.
Jet printers of the type to which the present invention is directed operate by selectively depositing small drops of ink at predetermined points on a print medium to form a print image. The drops of ink are formed from fluid filaments which emerge from small orifices communicating with an ink fluid reservoir in which ink is maintained under pressure. Each fluid filament tends to break apart at its tip to form a stream of ink drops. If no mechanical stimulation were applied, the drops would tend to be somewhat irregular in size and spacing within the stream. As will be appreciated, in order for an image of high resolution to be formed upon a print medium by selective deposition of the ink drops, it is necessary for the drops to be substantially uniform in size and for the interdrop spacing within the stream to be substantially uniform.
One technique utilized to enhance the regularity of drop size and spacing is to position the orifices in a relatively thin, flexible wall of the fluid reservoir. This wall, termed the "orifice plate," is then stimulated mechanically, causing a series of bending waves to travel along the wall. Such a stimulation technique is shown in U.S. Pat. No. 3,739,393, issued June 12, 1973, to Lyon et al in which dampers are positioned at each end of the orifice plate to reduce reflection of the bending waves. This technique, known as traveling wave stimulation, results in substantially uniform drop size and spacing. In a multiple jet printer using this technique, break up of drops along the print head will occur at various times, as the bending waves travel along the orifice plate. The formation of drops in each stream in such a printer is out of phase with respect to the formation of drops in the other streams. Additionally, the selection of orifice plate material for such a printer is drastically limited, since the plate must be sufficiently flexible in order to transmit the bending waves along the complete extent of the print head with sufficient amplitude.
Another approach to stimulation of jet drop break up is shown in U.S. Pat. No. 3,586,907, issued June 22, 1971, to Beam et al. This patent shows a printer in which the entire print head, including the ink manifold structure and orifice plate, is mechanically stimulated together. Such an arrangement will necessarily fatigue the print head mounting structure, since this structure will also experience the vibrations applied to the manifold and orifice plate.
A further approach to filament stimulation is disclosed in U.S. Pat. No. 4,095,232, issued June 13, 1978, to Cha. Using the technique disclosed in this patent, stimulators mounted in the upper portion of the fluid reservoir generate pressure waves which are transmitted downward through the fluid in the reservoir into the individual fluid filaments. Each stimulator includes a pair of piezoelectric crystals which vibrate in phase and which are mounted on opposite sides of a mounting plate which is coincident with a nodal plane. A reaction mass is positioned at the opposite end of the stimulator from a stimulation member which is coupled to the fluid. The reaction mass insures that the nodal plane is properly positioned. Such fluid coupled stimulation may, however, result in reflected plane waves from the orifice plate or other structure, which waves may limit in some instances the stimulation effectiveness of the arrangement.
Another stimulation technique is suggested in U.S. Pat. No. 3,667,678, issued June 16, 1972, to Haskell. In the Haskell patent, a nozzle structure is disclosed in which a relatively long tube defining the nozzle is supported at nodal points. The nozzle is stimulated by means of a magneto-strictive stimulator arrangement. The ink reservoir communicating with the tube, however, is fixed and the magneto-strictive stimulator arrangement is mounted by a separate mounting structure from that supporting the tube. The reaction force applied to the stimulator arrangement will, therefore, be coupled to other .[.part.]. .Iadd.parts .Iaddend.of the system through the stimulator support structure.
In U.S. Pat. No. 3,927,410, issued Dec. 16, 1975, to Pimbley, a nozzle, a vibrating means, and a reaction mass, are commonly supported for co-operative vibration. The supporting structure includes a rubber damper which is apparently not located at a nodal plane. Thus, while the rubber damper may reduce the amplitude of the vibrations transmitted to the rest of the printer, these vibrations will not be completely eliminated.
U.S. Pat. No. 3,683,396, issued Aug. 8, 1972, to Keur et al., discloses an ink jet system in which the length a tube shaped nozzle is selected such that it is in odd multiple of a quarter wave length of the frequency of vibration of the fluid in the nozzle. This configuration is said to optimize transmission of energy to the print fluid. The support structure for the nozzle, however, is not disclosed.
It should be appreciated that the above-referenced Haskell, Pimbley, and Keur et al patents all relate to stimulation of a single jet, which is much simpler to accomplish than multiple jet stimulation. In a multiple jet printer, vibrations transmitted through the mounting structure may affect stimulation of various ones of the jets, thus producing non-uniformity in the effectiveness of jet stimulation.
Accordingly, there is a need for a simple effective stimulator for an ink jet printer in which the mechanical vibrations applied to the fluid filaments are effectively decoupled from the printer support structure.